PASTE SUPPLEMENT 2019
and control pipelines which may have a
deposit.
Tailings pipeline leak detection
Figure 3: Laminar flow tailings discharge
slurries in laminar flow in horizontal pipelines is:
In laminar flow, there are two distinct zones: un-
sheared core where the yield stress exceeds the
local shear stress and the sheared annulus
where the shear stress exceeds the yield stress.
In the un-sheared core, providing the yield stress
is high enough, coarse particles are supported
by the yield stress and they do not settle. As the
yield stress has been overcome in the sheared
annulus, coarse particles settle in this region.
As coarse particles settle:
o The rheology in the sheared zone towards
the bottom of the pipe increases due to the
increased concentration of coarse particles
(rheology augmentation effect) which have
settled from higher in the pipe cross section.
o At the same time the rheology in the upper
portion of the pipe cross section decreases
due to the reduced concentration of coarse
particles.
n The above mechanism results in a non-
homogenous laminar flow: the center of the
un-sheared core is above the pipe axis and the
rheology within the sheared zone increases
towards the invert.
n If the wall shear stress at the pipe invert exceeds
the yield stress of the mixture at the bottom of
the pipe, the mixture will continue to flow (i.e.
particles will not deposit on the pipe invert).
n An equilibrium condition is established with
the settling force (immersed weight) acting on
the coarse particles being balanced by the
dispersive forces (shear rate, concentration
gradient and rheology gradient).
From the above description, it is evident that
the pre-condition for stable laminar flow is that
the yield stress of the material at the bottom of
the pipe does not exceed the local pipe wall
shear stress. The following aspects are
important:
Talmon et al. (2014) proposed the concept of a
“gel-bed” to estimate when stable laminar flow
conditions exist. The formation of a suitable gel-
P6 International Mining | APRIL 2019 Supplement
bed is highly dependent on the rheology of the
carrier fluid; for tailings slurries it is likely the
tailings need to contain a reasonable amount of
clay minerals to form a stable bed. It is important
to recognise that not all tailings are suitable for
transportation in laminar flow.
As the condition is dependent on the
relationship between local mixture yield stress
and pipe wall shear stress, it is possible that
slurries which are stable in small diameter pipes
are not stable in larger pipes (where the wall
shear stress is likely to be lower for an
equivalent operating velocity).
Continuing research in this field (e.g. Treinen
(2017) and the Saskatchewan Research Council
of Canada) is improving our understanding of
laminar flows of slurries in pipelines. It is probable
that a robust design approach for these pipelines
will be developed in the relatively near future.
Deposit height detection
It will be extremely valuable for operators of the
following pipelines to have real time data
regarding the height of a deposit on the pipe
invert:
n Pipelines designed to operate in turbulent
flow with a deposit under certain low flow rate
operating conditions. As noted previously the
height of deposit should be limited to avoid
potential pipeline blockage should deposited
material be mobilised rapidly.
n Changes in process conditions or tailings
properties may result in pipelines designed for
laminar flow operating in unstable conditions
resulting in the accumulation of material on
the pipe invert.
In these cases, knowledge of the bed height
will allow operators to take proactive corrective
action to reduce the bed height. Ilgner and
Kruger (2018) have developed a non-invasive
sensor network which maps deposit height in
an operating slurry pipeline. Figure 4 shows an
example of the output using the technology.
This sensor system will significantly improve
the ability of operators to effectively monitor
Along with advances in design and
implementation philosophy, pipeline monitoring
is becoming more relevant to tailings pipeline
engineering and operation. A predominant
feature in long distance slurry pipelines and the
oil and gas industry, pipeline management and
leak detection systems should be considered for
shorter and on-site tailings applications. Beyond
monitoring for overland sections of tailings
pipelines through sensitive areas, there is
motivation to use these systems to mitigate risks
of undetected on-dam crest pipeline leaks
leading to dam failure. This is not limited to
tailings pipelines, but any process pipeline
located on the dam crest.
Reliable leak detection systems are predicated
on monitoring pressure, density, temperature
and flow rate instrumentation and equipment
status in identifying any events or changes to
steady-state operation. Filtering of these events
to distinguish between leak and false alarm is
further complicated with operational variability.
Often, tailings pipeline operating conditions are
subject to variability due to frequent discharge
location adjustments to control reclaim pond or
beach characteristics as well as concentrator
process changes and upsets. This inherent
operating variability is the primary challenge for
leak detection systems for tailings pipeline
applications.
The design approach for tailings pipelines is
evolving to include operation with a deposit and
in laminar flow. This is being matched by new
and better technology to monitor pipeline
operating conditions and to quickly detect
pipeline leaks to allow for rapid response. These
developments will result in lower cost designs
(capital and operating) and more reliable
pipeline operation. IM
*First presented at the Gecamin Tailings 2018
Conference in Santiago, Chile
References
Cooke, R. (2002) "Laminar flow settling: The potential
for unexpected problems", 15th Int. Conf. on Slurry
Handling and Pipeline Transport, Hydrotransport 15,
Canada, June.
Ilgner, H, C. Kruger (2018) “Non-invasive sensor network
to map stationary bed heights and moving dunes along
a pipeline”, Paste 2018 Seminar, Perth, Australia, April.
Talmon, A. M., Kesteren, W. G. M. Van, Sittoni, L., and
Hedblom, E. P. (2014). “Shear cell tests for
quantification of tailings segregation.” The Canadian
Journal of Chemical Engineering, 92(2), 362¬373.
Treinen, J.M. (2017) “Modelling transport and deposition
of coarse particles in viscoplastic tailings beach flows”,
PhD Thesis, University of Colorado.